Burner control system



Oct. 4, 1966 R. w. ELDRIDGE, JR.. ETAL 3,276,507

BURNER CONTROL SYSTEM Filed Feb,

a JJ. 4 www .m #we www Mmmm United States Patent Oliice 3,276,507 Patented Oct. 4, 1966 3,276,507 BURNER CONTROL SYSTEM Raymond W. Eldridge, Jr., St. Louis County, Mo., and

Robert C. Krump, Waterloo, Ill., assignors, by mesne assignments, to Emerson Electric Co., a corporation of Missouri Filed Feb. 1, 1965, Ser. No. 429,480 1 Claim. (Cl. 158-28) This invention relates to automatic control systems for fluid fuel burners which inclu-des safety means operative to permit the supply of fuel to the burner when there is combustion flame and to cut olf such supply in event cornbustion fla-me is not established or having been estab-lished is extinguished, and which safety means includes a photoconductive device and a solid state switching device.

The primary object of the invention is to provide a generally new and improved safety control system for fluid fuel burners incorporating instantly responsive, combustion lia-me detecting means and solid state switching means in a novel arrangement which provides -a particularly simple, economical, and reliable construction.

Further objects and advantages will appear from the following description when read in connection with the accompanying drawing.

The single ligure of the drawing is a diagrammatic illustration of a control system for a conventional, pressure-type oil burner.

Referring to the drawing, the primary elements of the system include an oil burner having a driving motor 12, a space thermostat 14, a motor relay having a winding 16 with two sets of normally open contacts 18 and 20, a normally closed thermal time switch 22 (safety switch) including an electrical resistance heater 24, a light sensitive element 26, a silicon-controlled rectifier 28 (SCR) having a control electrode 25 and a control network comprising the light sensitive cell 26, a control resistor 30 with sliding contact 32, and a xed major voltage dropping resistor 34, a spark igniter comprising electrodes 36 and a step-up transformer 38, a voltage step-down, power supply transformer 40 having a primary winding 44 and a secondary winding 42, and a pair of power supply leads 46 for connection to a suitable source of A.C. power.

The oil burner 10 includes the usual blower and fuel pump which, when driven by motor 12, supply atomized oil and combustion air to the burner nozzle where it is ignited by arcing across spark electrodes 36. The motor 12 and igniter transformer 38 are connected in parallel across the' power supply leads 46 and through normally open motor relay contacts 18, when closed, by leads 48, 50, 52, 54, and 56. The motor 12 and igniter 38 a-re therefore energized simultaneously when relay contacts 18 are closed.

The motor relay winding 16 is connected across the entire secondary winding 42 through a circuit which may be trace-d from the left side of secondary 42 as follows: a lead 58, thermostat 14, a lead 60, a lead 62, safety switch 22, a lead 64, relay winding 16, a lead 66, a lead 68, safety switch resistance heater 24, a lead 70, SCR 28, and a lead 72 to the right side of transformer secondary 42.

The light sensitive element 26 is of the semiconductor type having an extremely high electrical resistance in the absence of light and becoming considerably less resistive when impinged by the light of burner flame. The positioning of light sensitive element 26 so as to be responsive to burner ame and yet be shielded from ambient light is preferably that shown and ldescribed in United States Letters Patent, No. 3,079,982, issued March 15, 1963, to Bernardus J. Staring.

The resistors 30 and 34 are series connected across the SCR 28, and the SCR control electrode 25 is connected to the left side of transformer secondary winding 42 through a lead 74, a lead 76, and slider contact 32, a portion of the control resistor 30, the major voltage dropping resistor 34, a lead 60, thermostat 14, and a lead 58. The light sensitive cell 26 and a portion of control resistor 30 are connected in parallel between the SCR control electrode 25 and the cathode side of the SCR by the lead 74 and by lead 76 and slider 32 and by leads 78, 80, and 82. The resistance between the SCR control electrode 25 and the left side of transformer secondary 42 due to resistor 34 and a portion of resistor 30 is such that suiicient or just slightly more than sufficient voltage is applied to the SCR control electrode during the 4conductive half cycle of the SCR and upon closure of thermostat 1-4 to effect conduction of the SCR under conditions of no flame `at the burner 10 when the resistance of light sensitive element 26 is very high. Upon the appearance of combustion flame at burner 10, the resistance of element 26 decreases and the voltage drop thereacross which then occurs is such that the voltage applied to the SCR control electrode falls below that which will effect conduction of the SCR.

The motor relay winding 16 is also connected `across a lefthand portion A of the transformer secondary winding 42 by a circuit branch extending from a point B at the junction of leads 60-62 and comprising the lead `62, the safety switch 22, lead 64, motor relay Winding 16, lead 66, a lead 84, relay contacts 20, and a lead 86 connected to the transformer secondary winding 42 at an intermediate point therealong.

The motor relay winding 16 and secondary winding 42 are so constructed, and the connection of lead 86 with winding 42 is so positioned therealong, that the half wave energization of relay winding 16 as effected by its connection across the entire secondary winding 42 through SCR 28 is sufficient to effect the closing of relay contacts 18 and 20, while, on the other hand, the full wave, A.C. energization of winding 16 as effected by its connection across the portion A of transformer secondary 42 is insuicient to cause the operation of the relay from open to closed contact position, but is sufficient to hold the relay in closed contact position once it is in closed contact position.

Operation Under normal operating conditions, the closure of thermostat 14 upon call for burner operation effects halfwave energization of relay winding 16 through safety switch 22, safety switch heater 24, and SCR 28 across the entire secondary winding 42 to cause the closing of contacts 18 and 20. The resistance of light sensitive element 26 under these conditions of no flame is very high, and a firing signal is therefore applied to SCR 28 each conducing half cycle. The closing of relay contacts 18 completes the circuit across power supply leads 46 thereby energizing burner motor 12 and igniter 38, whereupon fuel and air are supplied to the burner and ignited. The closing of relay contacts 20 completes the circuit branch connecting the relay winding 16 through the safety switch 22 (but not through the safety switch heater 24) across the lefthand portion A of transformer secondary winding 42.

When combustion flame appears at the burner 10, conduction of the SCR 28 is cut off and the relay contacts 18 and 20 remain closed due to the described branch circuit connecting relay winding 16 across portion A of the secondary winding 42. As stated hereinbefore, the power supplied to relay winding 16 by portion A of secondary winding 42, while suicient to hold relay contacts 18-20 closed once they have been closed, is insufcient to effect the closing of these contacts. When condu-ction through SCR 28 is cut off due to the appearance of burner flame, the safety switch heater 24 is de-energized and the burner 3 will continue to operate under normal conditions until thermostat 14 opens.

If upon closure of thermostat 14 and energization of burner motor 12 ignition of thecombustible mixture fails to occur within a predetermined time, or having been ignited is extinguished for any reason and fails to re-ignite in a predetermined time, the safety switch heater 24 will warp the bimetal contact blade of the safety switch 22 `and effect opening of the safety switch contacts, thereby breaking both the circuit connecting relay winding 16 across the entire secondary winding 42 and the branch circuit connecting it across the portion A of secondary winding 42. Under these conditions operation of the burner and igniter are cut olf, as Well as energization of the safety `switch heater 24 land conduction through SCR 28. A spring latching device 88 is provided which is operative to retain safety switch 22 in an open position when it is warped open so that manual resetting of the safety switch is required to kre-start the system.

If the SCR 28 malfunctions in a manner so as to conduct when it should not, or if the light sensitive element 26 fails to respond to burner flame and cut off conduction of SCR 28, the safety switch will open, the safety switch heater 24 being in series with the SCR 28. If SCR 28 malfunctions in a manner so as to fail to conduct when it should, or if light sensitive element 26 malfunctions or is inadvertently exposed to ambient light so as to prevent the application of a signal voltage to the SCR gate, the system will fail safely inasmuch as, under these conditions, relay winding 16 will not be energized suflciently to close contacts 18-20.

In event of a power supply failure Iduring operation of the burner, the relay winding 16 will be required to be energized through the safety switch heater 24 upon recurrence of the power supply, in order to again close contacts 18-20, so that continued operation of the burner motor is always contingent upon successful ignition within the predetermined trial period. The purpose of the slider contact 32 on control resistor 30 is to permit optimum calibration of the SCR gate network for light conditions of the particular installation and to compensate for variations in the Iresistivity of light sensitive element 26. Moving the slider contact 32 toward the anode side of the SCR, or left side of secondary -winding 42, increases the signal voltage on the SCR gate While moving it toward the cathode side of the SCR, or right side of secondary winding 42, decreases the signal voltage when the resistance of light sensitive element 26 is constant.

It is to be understood that the power supply voltage step-down transformer 40 may be dispensed with if a lower voltage than the available power supply is not required or desired. In such an larrangement the leads 58 and 72 are connected to the respectively adjacent leads 46, and the branch circuit lead 86 is connected tothe righthand lead 46 through a. suitable impedance.

The foregoing description is intended to be illustrative and not limiting, the scope of the invention being set forth in the appended claim;

We claim:

In a burner control system, a burner, an A.C. power source, a power source transformer having a secondary winding, a normally open relay operative when closed to cause fuel to be supplied to said burner and to cause the operation of ignition means to ignite it, a relay winding, a circuit connecting said relay winding across said secondary winding which when completed effects sucient half wave energization of said relay to close it, said cir- -cuit including in series relationship and in the order recited a space thermostat, a normally closed safety switch, said relay winding, .la resistance heater which when energized for a predetermined short time effects opening of said safety switch, and a silicon-controlled rectier having a control electrode, a flame responsive control network for said rectifier including a light sensitive element and a control resistor connected in parallel between said control electrode and the cathode side of said rectifier, and a voltage dropping resistor connected between said control electrodel and the anode side of said rectier at a point in said circuit between said safety switch and said thermostat, said light sensitive element being responsive to burner flame and having considerably higher electrical resistance in the absence of flame than when flame exists, the electrical resistance values of said resistors and of said light sensitive element in the absence of flame being such as to effect conduction of said rectifier, and the vdecrease in resistance of said light sensitive element in response to `burner ame being such as to reduce the signal applied to said control electrode below that required to effect conduction, and a branch circuit completed upon closure of said relay connecting said space thermostat, said safety switch `and said relay winding across such portion of said transformer secondary winding as will effect sucient full wave energization thereof to hold said relay closed, but insufficient to effect its closing.

References Cited by the Examiner JAMES W. WESTHAVER, Primary Examiner. 

